As telecommunications technology advances, and as the market for this technology expands, demand for bandwidth in networks is rapidly increasing. This bandwidth may be scarce or expensive. As a result, there is motivation to use as little bandwidth as possible for any given connection. One way to minimize the bandwidth used for a connection is to compress the data signals transmitted along the connection using standard compression algorithms. Such compression makes additional bandwidth available for additional connections. Compression may be performed on a wide variety of data signals, including but not limited to multimedia signals such as voice, video, and facsimile. Each time a signal is compressed and decompressed, some data may be lost, and the quality of the signal may be reduced. A single compression and decompression does not usually result in noticeable signal degradation. However, if a given data signal is compressed and decompressed multiple times, a scenario referred to herein as xe2x80x9ctandem encodings,xe2x80x9d there can be significant and noticeable degradation in the quality of the data signal.
Compression is particularly important in certain contexts, such as where bandwidth is scarce or expensive. For example, wireless transmissions require bandwidth on the wireless spectrum, which is available in limited amounts, and is increasingly expensive. As a result, voice transmissions over the wireless spectrum are typically compressed by a factor of 8:1 or more. Also, bandwidth is typically expensive on international networks, such that compression can lead to significant savings. Personal computers may not be able to handle bit rates above a particular bit rate. For example, 64 kB speech signals are presently beyond the capabilities of many personal computers, and speech signals to or from such a computer may be compressed by a factor of about 10:1. A data signal may also be compressed for storage in a device that has limited capacity. For example, voicemail is typically compressed by a factor of about 4:1 for storage in customer premises equipment (CPE). Answering machines may similarly compress voice signals to increase the capacity of the memory in the device.
Voice signals are sometimes compressed for transmission across a network. Depending on the algorithm used, there may be little or no perceptible degradation of voice quality when compared to the uncompressed 64 kbps pulse code modulation (PCM) format used in conventional telephone networks. However, there is frequently significant degradation of voice signals that have undergone tandem encoding. Thus, while it may be beneficial to compress a voice signal if bandwidth is scarce, care should be taken to avoid tandem encodings.
Moreover, the networks of different carriers are being interconnected with increasing frequency. For example; long distance carriers may be interconnected with local telephone networks, such that the voice signal of a long distance telephone call is typically transmitted from the caller, across the caller""s local telephone network, across a long distance carrier""s network, across the called party""s local telephone network, to the called party. While a given network may desire to minimize the use of bandwidth by compressing data signals, there is a risk of tandem encodings because the data signal may have previously been compressed by another network or another node within the given network. There is therefore a need for procedures to communicate, between networks and between nodes within a network, whether a data signal has been compressed, and if so, the type of compression.
Existing technology enables networks to transmit data signals to other networks, and existing signaling protocols allow the networks to communicate with each other about the data signals. Under the Broadband ISDN User Part (B-ISUP) protocol, for example, a first asynchronous transfer mode (ATM) network (1) transmits to a second ATM network an Initial Address Message (IAM) that indicates the final destination of the data signal; (2) receives an Address Complete Message (ACM) from the second network that indicates that a virtual circuit has been reserved, and the called party is being alerted; and (3) transmits the data signal after the called party answers, and the virtual connection has been established. There are many other protocols having similar procedures. For example, the PNNI protocol has an analogous procedure where SETUP and ALERTING messages replace the IAM and ACM, respectively. However, existing signaling protocols do not indicate whether and how a data signal has been compressed.
In a circuit-switched network, the bandwidth allocated per connection is either fixed or an integral multiple of some xe2x80x9cbasexe2x80x9d rate. For example, in a typical circuit-switched voice network in the United States, -connections are established in 64 kbps slices, though they could, in theory, be established as super-rate connections (i.e., as integral multiples of 64 kbps) or sub-rate connections (i.e., using a xe2x80x9cbasexe2x80x9d rate of less than 64 kbps). Even if a connection in such a typical network only requires 6 kbps of bandwidth, it would reserve a full 64 kbps of bandwidth through the network. As a result, compression does not necessarily reduce the use of bandwidth in a circuit-switched network.
In a packet network, the allocation of bandwidth to connections is continuously variable, and bandwidth usage may be finely controlled on a per-connection basis. As used herein, the term xe2x80x9cpacketxe2x80x9d refers to either a variable length packet or a fixed length cell. In a connectionless packet network, such as a transmission control protocol/Internet protocol (TCP/IP) network, the term xe2x80x9cconnectionxe2x80x9d is used herein to represent the transmission of packets comprising the data signal between two users across the network, possibly via a variety of different paths. As a result, a connection in a packet network ideally uses only the amount of bandwidth actually required. For example, a service that requires 6 kbps of bandwidth only reserves 6 kbps of bandwidth through a packet network, whereas the same service in a typical 64 kbps circuit-switched network would reserve 64 kbps. As a result, compression is usually beneficial in a packet network, because any bandwidth saved by compressing the data signal transmitted over one connection can be used for other connections. Bandwidth can therefore be used more efficiently in a packet network than in a circuit-switched network. Asynchronous transfer mode (ATM) networks and TCP/IP networks are examples of packet networks. The need for procedures to communicate compression information is therefore particularly acute when one or more packet networks are involved, because such information would allow more efficient use of bandwidth without signal degradation.
The present invention provides a method of transmitting a data signal across an intermediate network. The intermediate network receives a compression control signal associated with the data signal that provides information about the compression format of the data signal. The intermediate network uses this information to determine whether and where to transcode the data signal within the intermediate network. The data signal is then transmitted across the intermediate network, and transcoded as determined. As used herein, the term xe2x80x9ctranscodexe2x80x9d refers to the process of converting a signal from one (possibly compressed) format to another.
The present invention further provides another method of transmitting a data signal across an intermediate network. The intermediate network receives a first compression control signal from an originating entity that provides information about a first compression format in which the originating entity communicates with the intermediate network. The intermediate network also receives a second compression control signal from a terminating entity that provides information about a second compression format in which the terminating entity communicates with the intermediate network. The intermediate network uses the information provided by the first and second compression control signals to determine whether and where to transcode the data signal within the intermediate network. The intermediate network then transmits the data signal across the intermediate network, between the originating entity and the terminating entity, transcoding as determined.
The present invention further provides, in an entity, a method of transmitting a data signal between the entity and an intermediate network. The entity transmits a compression control signal, associated with the data signal, to the intermediate network. The compression control signal provides information about a compression format in which the entity communicates with the intermediate network. The data signal is then transmitted between the entity and the intermediate network in the compression format.
The present invention further provides an intermediate network adapted to transmit a data signal between an originating network and a terminating network. The intermediate network has an intermediate access switch connected to an originating switch of the originating network via a first link, and an intermediate egress switch connected to the intermediate access switch by a connection within the intermediate network, and connected to a terminating switch of the terminating network via a second link. The intermediate network is adapted to receive a compression control signal associated with a data signal, the control signal providing information about the compression format of the data signal. The intermediate network is further adapted to determine whether and where to transcode the data signal within the intermediate network, based on the compression control signal. The intermediate access switch is adapted to transmit the data signal between the intermediate access switch and the originating switch via the first link. The intermediate network is adapted to transmit the data signal across the intermediate network, transcoding the data signal as determined. The intermediate egress switch is adapted to transmit the data signal between the intermediate egress switch and the terminating switch via the second link.
The present invention further provides a switch adapted to practice the methods of the invention, and a medium that stores instructions as to how to practice the methods of the invention.